Grease is widely used in lubrication of various types of machinery such as automobiles, electric machines, construction machines, production lines, information equipment, industrial machines, machine tools, etc., and also members making up the above-mentioned machinery. As lubricants for use in severe circumstances, for example, at high temperatures or low temperatures, and under high loads, fluorine-based greases comprising a perfluoropolyether base oil, a fluororesin, and various additives have been widely used.
With recent trends of higher speed, smaller sizes, higher performances and lighter weight of machines, various additives are added to these fluorine-based greases to correspond to more and more severe service condition. For example, fluorine-containing organophosphorus compounds are known as fluorine-based additives having improved effects on solvent resistance, chemical resistance, mold releasability, abrasion resistance, friction resistance, etc., and in this connection, the present applicants have so far proposed a lubricating oil composition having distinguished abrasion resistance and rust prevention, based on a perfluoropolyether base oil containing a specific phosphonic acid compound, as a fluorine-based base oil. However, since phosphonic acid group resides only at one terminal of the molecule, the proposed lubricating oil composition is hard to satisfy the recently imposed requirements for higher lubricability and rust prevention at the present time.
Patent Literature 1: JP-A-2003-027079
Besides the fluorine-based additives, solid lubricants such as graphite, molybdenum disulfide, boron nitride, etc. are known to improve the lubricability, and addition of organic molybdenum compounds has been proposed to improve the abrasion resistant characteristics. However, these additives each have their own specific drawbacks. For example, graphite is chemically stable and less expensive, and has a distinguished lubricability, but any satisfactory rust prevention cannot be obtained even by adding it to perfluoropolyether base oil and fluororesin, and furthermore due to the black color peculiar to its nature, the graphite is intentionally not used with the exception of specific applications.
The following Patent Literature 2 discloses a method for forming an oxide film on the rolling surface of a bearing upon admixing a passivating oxide, but there is such a problem that any satisfactory effect cannot be attained unless the oxide film can be thoroughly formed. Furthermore, it is hard to completely satisfy the recently imposed requirements for higher heat resistance, abrasion-resistant characteristics, friction-resistant characteristics, and rust prevention.
Patent Literature 2: Japanese Patent No. 2,878,749